Cable testers are typically used to detect problems in a cable (e.g., a Category 5 cable and so on). For example, a cable tester can perform an analysis of a cable to detect a short, an open, a crossed pair, or a reversed pair within the cable. A short condition occurs when two or more wires within a cable are short-circuited together. An open condition occurs when there is a lack of continuity between ends at both ends of a cable. A crossed pair occurs when a pair of wires within a cable communicates with different pins at each end of the cable. A cable tester can also be used to determine a length of a cable. A cable tester can be coupled to one end of a cable through a first connector (such as an RJ-45 or other connector), and a second end of the cable can be connected to a load through a second connector. Conventional cable testers typically require the load to be a remote node terminator or a loop back module, as conventional cable testers may generate inaccurate results when a cable is terminated by an active link partner that is generating link pulses during a test.
Cable testers can employ time domain reflection (TDR), which is a measurement technique used to determine the characteristics of electrical lines by observing reflected waveforms. The amplitude of the reflected signal can be determined from the impedance of a discontinuity. For example, a cable tester can transmit a test pulse on a cable and measure an elapsed time until a reflection from the cable is received. Using the elapsed time and a cable propagation constant, a distance to a fault within the cable can be estimated. During a cable test, two waves can propagate through a cable—a forward wave that propagates from a transmitter in the cable tester towards the load or fault, and a return wave (or reflection) that propagates from the load or fault to the cable tester.
A perfectly terminated line has no attenuation and an impedance that is matched to a source impedance—i.e., the load is equal to the line impedance. The return wave is zero for a perfectly terminated line because the load receives all of the forward wave energy. For open circuits, the return wave has an amplitude that is approximately equal to the forward wave. For short circuits, the return wave has a negative amplitude that is also approximately equal to the forward wave.
Ethernet network devices such as switches, routers, and network appliances typically include physical layer devices with ports that connect to a cable. Cable tester technology that is implemented with a physical layer device (PHY) of an Ethernet network device allows the PHY to check a quality of a cable connection without the need to unplug the cable, insert a cable tester, and loop back the far ends of the cable.